1. Field of the Invention
The present invention relates generally to heat resistance fiber obtained from polyamide-imide resins and, more particularly, heat resistant fiber having tenacity of at least about 1.5 grams per denier, and elongation of at least about 10%, solution-spun from amide-imide resin comprising the polymeric condensation product of reactants comprising at least one member selected from the group of reactant pairs consisting of (a) trimellitic anhydride and toluene diisocyanate and (b) trimellitic anhydride chloride and toluene diamine. The TMAC-TDA and TMA-TDI polyamide-imide resins used to obtain the fiber of the present invention are defined in terms of specified inherent viscosity and molecular weight values which we have discovered are critical for obtaining resin which can be solution-spun into the fiber of the present invention. Accordingly, the fiber of the invention is obtained from resin comprising amide-imide repeating units of the following general formula: ##STR2## wherein the resin has an inherent viscosity of from about 0.3 to about 1.3 dl/g; an M.sub.n of at least about 5000 g/mole; a ratio of M.sub.w /M.sub.n in the range of from about 1.7 to about 3.3; and a ratio of M.sub.z /M.sub.w of not greater than about 2.3. The fiber of the present invention has excellent thermal stability and can be incorporated in a wide variety of textile, paper or other fiber-containing products, both woven and non-woven, to impart heat and flame resistance.
2. Discussion of Background Art
There exists a strong need for heat-resistant fibers useful in the manufacture of protective clothing, high temperature filtration fabrics, electrical insulation paper, honeycomb construction used in aircraft, and a wide variety of other products in which resistance to heat is essential. Aromatic polyamideimides are potentially well suited as resins for spinning heat-resistant fibers due to their high glass transition temperatures (typically above 250.degree. C.), thermal oxidative stability, and inherent flame resistance. Although it may be possible in rare instances to achieve melt spinnability of aromatic polyamide-imide resins, resins having the greatest potential for flame resistant products generally are found to decompose before melting and must therefore be solution-spun. As is generally well known, solution-spinning is carried out by dissolving the fiber-forming resin in an appropriate solvent to obtain a spinning solution. The solution or "dope" is then forced through a spinneret into a coagulation bath in the case of wet solution-spinning, or into a gaseous medium in the case of dry solution-spinning. The organic liquid chosen for the coagulation bath, or the gaseous medium in the case of dry spinning, must be such that the dope solvent will dissolve or evaporate into it, but the polymer will not. As the dope solvent transfers out of the dope, fiber is formed.
Preparation of aromatic polyamide-imides from either trimellitic anhydride and aromatic diisocyanates, or from trimellitic anhydride chloride and aromatic diamines is well known in the literature, as are techniques for wet or dry solution-spinning of such resins into fiber. For example, R. Pigeon and P. Allard, in a published lecture entitled "Heat-Resistant and Flame-Resistant Fibers" (Die Angewandte Makromolekulare Chemie, Vol. 40/41, No. 600, pp 139-158, 1974) investigated the direct polycondensation reaction in polar solvent of trimellitic acid anhydride with different aromatic diisocyanates, as well as the solution-spinnability of the resulting polyamide-imide resins. In discussing the influence of differing diisocyanates on the polycondensation reaction with trimellitic acid anhydride, the authors state that the results of the polycondensation, as well as the spinning suitability of the obtained polymer, depend upon the reactivity of the diisocyanate as well as the solubility of the corresponding polymer. Although in Table 2 of this paper the authors disclose polyamide-imides prepared from trimellitic acid anhydride and the 2,6 or 2,4 isomers of toluene diisocyanate, they teach that only diisocyanates with two benzene nuclei (preferably diphenylmethane diisocyanate and diphenyloxide diisocyanate) have satisfactory reactivity, and produce resin of satisfactory solubility for solution-spinning. The data which the authors present in Table 2 of the paper reinforce this teaching insofar as no fiber properties (i.e., tenacity or elongation data) are given for the polyamide-imide prepared from toluene diisocyanate, whereas fiber data are shown for the polyamide-imides based on diphenylmethane diisocyanate and diphenyloxide diisocyanate. Considered as a whole, the Pigeon and Allard paper fails to teach a polyamide-imide fiber based upon trimellitic acid anhydride and toluene diisocyanate, particularly in view of the authors' statement that only diisocyanates with two benzene nuclei are satisfactory.
Co-author P. Allard of the above-mentioned technical publication is also the inventor of U.S. Pat. Nos. 3,929,691 and 3,903,058 and a co-inventor of Rochina et al. U.S. Pat. No. 3,717,696, all of which deal with polyamideimide resins based on the polycondensation products of trimellitic acid anhydride and aromatic diisocyanates, and the solution-spinning of heat-resistant fibers therefrom. Consistent with the teachings found in the technical publication of Pigeon and Allard, discussed above, these patents disclose a clear preference for polyamide-imides based on the reaction of trimellitic acid anhydride with an aromatic diisocyanate having two benzene nuclei. Notwithstanding the fact that the '696 and '058 patents disclose the suitability of mono- as well as bi-nuclear aromatic diisocyanates, the examples of all three of the above-mentioned patents are limited in their teaching of possible diisocyanates to 4,4' diisocyanatodiphenylmethane (sometimes referred to as methyl diphenyl isocyanate or "MDI") and 4,4 diisocyanatodiphenylether (sometimes referred to as oxydiphenyl isocyanate or "ODI").
The above-mentioned Allard '691 patent discloses wet or dry spinnable solutions of high molecular weight polyamide-imide copolymers in an anhydrous solvent which is inert to the copolymers, where the copolymers are derived from aromatic diisocyanates which contain two benzene nuclei (preferably ODI and MDI), an aromatic anhydride acid (preferably trimellitic acid anhydride), an aromatic or heterocyclic diacid (preferably isophthalic or terephthalic acids) and, optionally, a dianhydride (preferably pyromellitic dianhydride); and where the copolymers have an inherent viscosity between 0.5 and 1.6 dl/g, as measured by using a 0.5% strength solution thereof in N-methylpyrrolidone. The patent discloses conducting the copolymerization in the solvent such that the reaction temperature is progressively raised during the reaction from 25.degree. to 100.degree. C. at the beginning of the reaction to 120.degree. to 250.degree. C. at the end of the reaction. This patent does not disclose a fiber spinning solution in which the amide-imide polymer of the solution is derived from trimellitic acid anhydride and aromatic diisocyanates containing only one benzene nucleus, such as toluene diisocyanate.
The above-mentioned Rochina et al. '696 patent discloses a process for producing polyamide-imide filaments by dry spinning a solution of polyamideimide under specified conditions. In a preferred embodiment, the polyamideimide polymers in solution are extruded into filaments and fibers through a spinneret maintained at a temperature between 60.degree. C. and 180.degree. C.; then the filaments are heated at a temperature higher than about 160.degree. C. up to about 240.degree. C. at a constant length for 2 to 6 hours; and subsequently drawn at a drawing ratio of at least 3:1 at a temperature generally in the range of about 220.degree. C. up to about 420.degree. C. The patent states that thermal treatment of the fiber prior to drawing causes a substantial increase in the tensile strength of the drawn filaments. The patent further states at column 2, lines 38-41, that the polyamide-imides used in the invention must have an inherent viscosity greater than 0.4, and preferably from 0.8 to 1.4, as measured at 25.degree. C. on a 0.5% weight for volume solution in the solvent used in the preparation of the polyamide-imide polymer. The patent discloses for use as the spinning solution polyamide-imide solutions obtained by reacting in substantially stoichiometric proportions in a polar organic solvent at least one aromatic diisocyanate and an acid reactant containing at least an aromatic anhydride-acid (preferably trimellitic acid anhydride) and optionally also at least one di-acid such as terephthalic or isophthalic acid. Toluene diisocyanate is disclosed in the patent as among the suitable diisocyanates for preparing the polyamide-imide solution. The patent also points out that the polyamide-imide can alternatively be prepared by reaction of a diamine with the chloride derivative of the acid anhydride reactant. Despite the patent's mention of toluene diisocyanate, all of the patent's examples are limited to MDI and ODI, which are diamines having two benzene nuclei. The absence in the '696 patent of any example using toluene diisocyanate, or any other diamine or diisocyanate having only one benzene nucleus, is consistent with the teaching found in the co-inventor Allard's technical paper (discussed above) that only diisocyanates having two benzene nuclei are satisfactory for the production of fiber grade polyamide-imides.
The Allard et al. '058 patent, like the patents discussed above, is directed to heat stable fibers based on polyamide-imide resins which are the reaction product of reactants comprising aromatic diisocyanates and aromatic acid anhydrides. Again, while toluene diisocyanate is said to be a suitable reactant, there are no examples in the patent disclosing a fiber based on this reactant. The Allard '058 patent states that bright, homogeneous yarns can be obtained by wet spinning a solution containing a copolymer having both amide-imide and amide-acid groups.
Serres et al. U.S. Pat. No. 3,839,529 discloses preparation of polyamide-imide filaments based on the reaction product of an acyl halide derivative of trimellitic acid anhydride which contains at least one acyl halide group in the 4-ring position, with aromatic primary diamines in polar organic solvents at temperatures below 150.degree. C. The resulting products are polyamic acids which are then water precipitated, heated, dry spun and cured (preferably with drawing) to obtain continuous filaments. More particularly, the process of the invention involves (1) heating the precipitated polyamic acid at a temperature between about 300.degree. F. and 600.degree. F.; (2) dissolving the heated polymer into a polar organic solvent at such a concentration that the solution viscosity of the resulting solution is at least 1500 poise, preferably between 2000-2500 poise, when measured at 25.degree. C.; (3) spinning the polymer solution into a gaseous atmosphere which is maintained at a temperature of at least 450.degree. F.; and (4) curing the spun filaments at a temperature above 300.degree. F. for a time sufficient to convert substantially all of the carboxyl and amide groups available for further reaction to imide groups. According to the teachings of the patent, the tenacity of the fibers is enhanced by orienting (i.e., drawing) the fiber during the above mentioned curing step. The patent discloses, as useful diamines for preparation of the polyamic acids, wholly or largely aromatic primary diamines, particularly aromatic primary diamines containing from 6 to about 10 carbon atoms or aromatic primary diamines composed of two divalent aromatic moieties of from 6 to about 10 carbon atoms, each moiety containing one primary amine group, with the moieties linked directly or through bridging groups such as --O--, --CH.sub.2 --, --CO--, --SO.sub.2 --, and --S--. Polyamic acids or polyamide-imides based on toluene diisocyanate or toluene diamine are not specifically disclosed. The patent further states that the primary diamine reactant and the anhydride reactant are present in essentially equimolar amounts, and that variations of up to about 3 mole percent in either direction do not substantially affect the resulting polymer. Notwithstanding the many advantages disclosed in this patent, it is desired to avoid the separate disclosed step of precipitating the polyamic acid intermediate, as well as the step of heating the precipitated material or curing the fiber spun therefrom, which steps are utilized in the patent to convert the polyamic acid to polyamide-imide.
The ability to manufacture high quality heat-resistant fibers from polyamide-imide resin based on toluene diamine or toluene diisocyanate is highly desirable due to the lower cost of these reactants as compared with 4,4' diisocyanato- (or diamino-) diphenylmethane and 4,4' diisocyanato- (or diamino-) diphenylether. Moreover, the lower cost of toluene diisocyanate versus toluene diamine makes especially desirable the capability of producing high quality fiber from TMA-TDI resin. Nevertheless, despite considerable economic incentive, and notwithstanding the technical publication and patents referred to above, the art has not been able, to the best of our knowledge, to produce a high quality heat-resistant fiber from polyamide-imides based on the reaction of either trimellitic acid anhydride and toluene diisocyanate or trimellitic acid anhydride chloride and toluene diamine.
It is an object of the present invention to provide a heat-resistant fiber obtained from polyamide-imide resin based on the reactant pair of trimellitic acid anhydride and toluene diisocyanate and having commercially desirable tenacities and elongations and to provide novel articles of manufacture comprised of such fiber having excellent thermal, mechanical and aesthetic properties. Other objects will become apparent hereinafter to those skilled in the art.